1. Field Of The Invention
This invention relates to a new and improved form of crystalline silicate, to a new and useful improvement in synthesizing said crystalline silicate and to use of the crystalline silicate prepared in accordance herewith as a catalyst for organic compound, e.g. hydrocarbon compound, conversion.
More particularly, this invention relates to an improved method of preparing crystalline silicate having the structure of ZSM-50 of high silica/alumina mole ratio whereby synthesis is facilitated and reproducible and the silicate product exhibits high purity and catalytic utility.
2. Discussion Of Prior Art
Zeolitic materials, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversion. Certain zeolitic materials are ordered, porous crystalline aluminosilicates having a definite crystalline structure as determined by X-ray diffraction, within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels or pores. These cavities and pores are uniform in size within a specific zeolitic material. Since the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of larger dimensions, these materials have come to be known as "molecular sieves" and are utilized in a variety of ways to take advantage of these properties.
Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline aluminosilicates. These aluminosilicates can be described as a rigid three-dimensional framework of SiO.sub.4 and AlO.sub.4 in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total aluminum and silicon atoms to oxygen atoms is 1:2. The electrovalence of the tetrahedra containing aluminum is balanced by the inclusion in the crystal of a cation, for example an alkali metal or an alkaline earth metal cation. This can be expressed wherein the ratio of aluminum to the number of various cations, such as Ca/2, Sr/2, Na, K or Li, is equal to unity. One type of cation may be exchanged either entirely or partially with another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given aluminosilicate by suitable selection of the cation. The spaces between the tetrahedra are occupied by molecules of water prior to dehydration.
Prior art techniques have resulted in the formation of a great variety of synthetic zeolites. The zeolites have come to be designated by letter or other convenient symbols, as illustrated by zeolite A (U.S. Pat. No. 2,882,243), zeolite X (U.S. Pat. No. 2,882,244), zeolite Y (U.S. Pat. No. 3,130,007), zeolite ZK-5 (U.S. Pat. No. 3,247,195), zeolite ZK-4 (U.S. Pat. No. 3,314,752), zeolite ZSM-5 (U.S. Pat. No. 3,702,886), zeolite ZSM-11 (U.S. Pat. No. 3,709,979), zeolite ZSM-12 (U.S. Pat. No. 3,832,449) zeolite ZSM-20 (U.S. Pat. No. 3,972,983), ZSM-35 (U.S. Pat. No. 4,016,145), ZSM-38 (U.S. Pat. No. 4,046,859), and zeolite ZSM-23 (U.S. Pat. No. 4,076,842), merely to name a few.
The SiO.sub.2 /Al.sub.2 O.sub.3 ratio of a given zeolite is often variable. For example, zeolite X can be synthesized with SiO.sub.2 /Al.sub.2 O.sub.3 ratios of from 2 to 3; zeolite Y, from 3 to about 6. In some zeolites, the upper limit of the SiO.sub.2 /Al.sub.2 O.sub.3 ratio is unbounded. ZSM-5 is one such example wherein the SiO.sub.2 /Al.sub.2 O.sub.3 ratio is at least 5 and up to infinity. U.S. Pat. No. 3,941,871 (Re. 29,948) discloses a porous crystalline silicate made from a reaction mixture containing no deliberately added alumina in the recipe and exhibiting the X-ray diffraction pattern characteristic of ZSM-5 type zeolites. U.S. Pat. Nos. 4,061,724, 4,073,865 and 4,104,294 describe crystalline silicates or organosilicates of varying alumina and metal content.
The prior art techniques for synthesis of zeolites utilize reaction mixtures containing various organic directing agent. The directing agent is extremely important in such synthesis methods since it functions as a template in the nucleation and growth of the desired zeolite. Different directing agents direct nucleation and growth of different zeolites from even similar reaction mixtures. Even with the same directing agent, however, different zeolites will form occasionally, depending upon such factors as reaction mixture composition. Still further, the crystallization conditions, such as temperature and time, may affect product type and quality.
U.S. Pat. No. 4,391,785 teaches a method for synthesis of zeolite ZSM-12 from a reaction mixture comprising, as a directing agent, a compound selected from the group consisting of dimethyl pyridinium halide and dimethyl pyrrolidinium halide. U.S. Pat. No. 4,296,083 claims synthesizing zeolites characterized by a constraint index of 1 to 12 and an alumina/silica mole ratio of not greater than 0.083 from a specified reaction mixture containing an organic nitrogen-containing cation provided by an amine identified as being selected from the group consisting of triethylamine, trimethylamine, tripropylamine, ethylenediamine, propanediamine, butanediamine, pentanediamine, hexanediamine, methylamine, ethylamine, propylamine, butylamine, dimethylamine, diethylamine, dipropylamine, benzylamine, aniline, pyridine, piperidine and pyrrolidine. The latter patent mentions dibenzyldimethylammonium compounds as possibly useful sources of organic nitrogen-containing cations.
U.S. Pat. No. 4,151,189 claims a method for synthesizing zeolites ZSM-5, ZSM-12, ZSM-35 and ZSM-38 containing an organic nitrogen cation from a specified reaction mixture containing a primary amine having 2 to 9 carbon atoms as a directing agent. U.S. Pat. No. 4,112,056 teaches a synthesis method for ZSM-12 from a reaction mixture containing tetraethylammonium ions as directions agent. U.S. Pat. No. 4,452,769 claims a method for synthesizing ZSM-12 from a reaction mixture containing methyltriethylammonium ions as the directing agent.
A zeolite structure, called "EU-1" in European Patent Application No. 42,226, "TPZ-3" in European Patent Application No. 51,318 and "ZSM-50" in U.S. application Ser. No. 101,252, is shown to be synthesized from a reaction mixture comprising a hexamethylene diammonium salt directing agent.
U.S. Pat. Nos. 3,699,139 and 3,783,124 teach use of benzyltrimethylammonium ions as directing agents in synthesis of crystalline materials other than zeolite ZSM-50. U.S. Pat. No. 3,947,482 describes a method for synthesis of open pore zeolite such as offretite by hydrothermal reaction of a silica source, alumina source, sodium hydroxide and/or potassium hydroxide in the presence of a zeolite nucleation slurry synthesized from a mixture of a silica source, alumina source, sodium hydroxide and/or potassium hydroxide and an organic template, the template potentially being a benzyltrimethylammonium compound.
Applicant knows of no prior art methods for preparing crystalline silicate ZSM-50 utilizing the present improved method.